H.E.S.S.

High Energy Stereoscopic System

Another Shell-Type Supernova Remnant

RX J0852.0-4622 ("Vela Junior")

March 2005

Rosat image of the
Vela supernova remnant and its surroundings, for X-ray energies above 1.3
keV, showing the big Vela remnant, the small Puppis-A supernova remnant in
the upper right, and the in this image newly discovered remnant RX
J0852.0-4622 in the lower left (Aschenbach,
1998)

The supernova remnant RX J1713.7-3946 (see
Jan. 2005) was the first remnant where the shell structure was detected in VHE
gamma rays, demonstrating that the supernova shock wave accelerates particles.
The detailed interpretation is complicated by the fact that there is significant
uncertainty in a number of crucial parameters, such as the distance to the
remnant and the characteristics of the ambient interstellar medium. It is
therefore important to enlarge the ensemble of supernova remnants studied in VHE
gamma rays, which allows one to average over such parameters and to extract the
key features. "Vela Junior", or RX J0852.0-4622 is supernova remnant discovered
in 1998 in ROSAT X-ray images (Aschenbach
1998,
Aschenbach at al. 1999). The remnant is only a faint radio emitter (Duncan
& Green, 2000) and was not identified in earlier radio surveys. Age and
distance are estimated to 680 years and 200 pc (Aschenbach
at al. 1999). It has been argued that three nearby supernova explosions
might be responsible for spikes in the nitrate abundance found in South Pole ice
cores (Rood
et al, 1979); with an age around 700 years, the Vela Junior supernova might
be responsible for a fourth, previously unidentified spike (Burgess
and Zuber, 2000) (Fig. 1). The CANGAROO instrument
reported the detection at the 6 sigma level of VHE gamma rays from the
north-western part of Vela Junior, based on about 100 h of observations (Katagiri
et al., 2005).

With the H.E.S.S. telescopes, a clear signature of Vela
Junior in VHE gamma rays was detected in only 3.2 h of exposure, with a
significance of 12 sigma (Fig. 2). In the H.E.S.S. image,
a shell-like morphology is clearly visible, coincident with the X-ray morphology
of the remnant. The radius of the shell is almost 2 degrees, with a peak of VHE
emission in the north-western (upper-left) section of the remnant. The total
flux from the remnant is at the same level as the flux from the Crab Nebula,
which makes it one of the strongest galactic sources of VHE gamma rays. The
close resemblance between the X-ray image and the gamma-ray image is
demonstrated by a correlation coefficient of 0.7, obtained by dividing the
image into bins of 0.4 by 0.4 degrees. X-ray images show a compact X-ray source
at the center of the remnant (AX J0851.9-4617.4,
Slane et al. 2001), which could be a neutron star created in the explosion;
in the H.E.S.S. image, no significant gamma-ray excess from this object is
detected.

The differential gamma-ray spectrum obtained with the
H.E.S.S. telescopes is shown in Fig. 3, it follows a power
law with a spectral index of 2.1, in good agreement with the spectra predicted
for shock-wave acceleration of nuclei in the supernova shock wave. In such a
scenario, the VHE gamma rays result from interactions of accelerated nuclei with
the ambient interstellar medium. Assuming a local gas density of 1/cm3,
the total energy in accelerated nuclei is estimated to 1049 ergs,
which would imply a conversion efficiency of a few percent between kinetic energy
released in the explosion and energy of accelerated cosmic rays. With its large
size and high gamma-ray flux, Vela Junior is obviously a good target for more
detailed studies of the morphology of VHE gamma ray emission, and it is planned
to record larger data sets for this source.

References:

Fig. 1:
Nitrate abundance in South Pole ice cores. The two spikes at ~50 m correspond to the dates around 1600, the spikes at ~85 m and ~100 m to 1300
and 1150, respectively. The increases in Nitrate abundance have been
associated with Supernovae observed in 1604 (Kepler's supernova), 1572 (Tycho's
supernova), and in 1181. The 700 year old Vela Junior supernova might have
caused the fourth spike at ~85 m. See
Rood et al. and
Burgess and Zuber.